Trait variation across a plant hybrid zone 25 years in the making

In natural hybrid zones, traits often show clines, gradual variation across a gradient reflecting a balance between natural selection and gene flow. Changes over time in average values for traits, and especially the shapes of their clines, are rarely investigated in plants but could result from evolution in an unstable hybrid zone. Differences in clines between floral and vegetative traits could indicate different strengths of divergent selection.

Campbell et al. examine traits in 12 populations spanning a hybrid zone between Ipomopsis aggregata and I. tenuituba (Polemoniaceae), and for floral traits compare clines across two decades. Clines vary in steepness across traits, with both floral and vegetative traits showing steeper clines than molecular markers, consistent with selection on both suites of morphological traits.

Although cline shapes did not change, flowers are now longer as predicted by selection and heritability. The increase in corolla length provides a rare example of a match between the predicted and observed evolution of a plant trait in natural populations. The clinal properties are consistent with the hypothesis that habitat-mediated divergent selection on vegetative traits and pollinator-mediated selection on floral traits both maintain species differences across the hybrid zone.